This invention relates to a method for improving the machinability of titanium (Ti) and titanium alloys. It also relates to free-cutting titanium alloys and method for the preparation thereof.
More particularly, the present invention relates to a method for improving the machinability of titanium and titanium alloys which are suitable for use in parts such as structural members of vehicles, including aircraft and automobiles and movable members of the engines of these vehicles which are required to be light weight and of high strength.
Pure titanium and titanium alloys find applications in parts of high speed vehicles such as aircraft and automobiles due to their light weight and high strength. However, in the manufacture of such parts from titanium or a titanium alloy by machining, the poor machinability of the material limits the tool life and the machining speed. Therefore, the machining process is costly and time-consuming and the mass-production of titanium or titanium alloy parts has been difficult. This is one of the reasons for the high costs of titanium or titanium alloy products.
It has been known that the machinability of titanium and titanium alloys is inferior to that of steels. The poor machinability of titanium and titanium alloys is thought to result from (i) an increased force imposed on the edge of a cutting tool due to the mechanism of the formation of cuttings inherent in titanium and its alloys, which causes the edge to be readily damaged, (ii) an increased cutting temperature, i.e., the temperature in the cut area due to the lower thermal conductivity of titanium and its alloys compared to steel, and (iii) a higher susceptibility of titanium to reaction with the cutting tool than steel as evidenced by the fact that titanium is more reactive with other elements than steel.
Accordingly, there is a continuing need to improve the machinability of titanium and titanium alloys.
It has been proposed that the machinability of titanium and titanium alloys can be improved by adding one or more elements selected from S (sulfur), Se (selenium), Te (tellurium), REM (rare earth metals), and Ca (calcium) [Japanese Patent Application Kokai Nos. 60-251239(1985), 61-153247(1986), 61-257445(1986), and 62-89834(1987), U.S. Pat. No. 4,810,465, and European Patent Publication No. 199,198]. These elements form inclusions in titanium or a titanium alloy and act to improve the machinability thereof. However, since the addition of such elements simultaneously causes a decrease in hot workability and mechanical strength (particularly fatigue strength), the amounts of these elements which can be added are limited. As a result, the addition of S, Se, Te, REM, and/or Ca in limited amounts not only cannot provide the resulting titanium alloy with a satisfactory improvement in machinability, but also degrades the hot workability and fatigue strength of the titanium alloy so that it is inferior to a conventional titanium or titanium alloy in hot workability and fatigue strength.